A bright streak flashes from your pointing finger to a point you choose within range and then blossoms with a low roar into an explosion of flame. Each creature in a 20-foot-radius sphere centered on that point must make a Dexterity saving throw. A target takes 6d8 necrotic damage on a failed save, or half as much damage on a successful one. For the next 3 turns a creature who has been hit takes 1d8 necrotic damage at the beginning of their turns.